Cross-connect for an optical network

ABSTRACT

An optical cross-connect comprises a plurality of input modules (10 1  . . . 10 n , 12 1  . . . 12 n ) each for receiving a respective group of input signals (11 1  . . . 11 n , 13 1  . . . 13 n ) and deriving in response to each signal of the group an individual optical signal of a respective given wavelength, a plurality of output modules (15 1  . . . 15 n , 16 1  . . . 16 n ) each including a plurality of receptors (23, 24; 27, 28) selectively responsive to respective ones of the wavelengths of said optical signals, to provide an output signal, and a plurality of couplers (14 1  . . . 14 k ) each for coupling a respective one of said optical signals, each of a different wavelength, from each of the input modules, to a respective receptor of each of the output modules.

FIELD OF THE INVENTION

This invention relates to a cross-connect for an optical network.

BACKGROUND OF THE INVENTION

In order to cope with the increasing demand for subscriber services ontelecommunication networks, digital optical transmission networks arebeing introduced in which local access networks, which typically routeelectrical signals, are interconnected by a fibre optic network. Thishas been facilitated by the introduction of the Synchronous DigitalHierarchy (SDH) standard that deals with the formatting of signals inSynchronous Transfer Modules (STM) which will allow a unifiedtelecommunication infrastructure and provides improved flexibility bypermitting electrical digital signals from the local access networks tobe converted into optical signals and transmitted through the opticalnetwork.

In order to upgrade the capacity of the optical network,Wavelength-Division-Multiplexing (WDM) techniques have been proposed,which permit the transmission capacity of a fibre link to be upgraded tothe multi-Gbit/s range.

Thus, it has been proposed to provide a multi-Gbit/s WDM networksuperimposed on the top level of a SDH network. For a fuller discussion,reference is directed to I. Hawker, "Evolution of Digital OpticalTransmission Networks", BT Technol. J., Vol. 9, No. 4, October 1991, pp43-5. It is proposed to include nodes in the WDM network to allowdynamic re-routing of WDM data streams. It is proposed that each nodeshould include access ports to allow optical streams to be dropped toand added from lower levels of the network. It has been proposed thatrouting of the high bit rate streams is directly performed in theoptical layer whereas processing of any of the streams is achieved inthe electronic domain by dropping it to a lower layer of the network.Network management and administration is proposed to be achieved from amanagement unit which configures an optical cross-connect at each nodeaccording to traffic requirements. Thus, optical cross-connects (OXC's)constitute the routing nodes of the WDM transport network.

A number of prior proposals have been made for the node architecture ofthe OXC and an example is given in FIG. 1.

Three input fibres 1, 2, 3 are connected at the node to output fibres 4,5, 6. Each of the fibres 1-3 carries four WDM wavelength channels, whichare connected to the output fibres 4-6 by means of space switches X₁-X₄. In this example, the space switches are 4×4 matrices which thushave four inputs a-d and four outputs e-h that are opticallyinterconnectable and are controlled by applied electrical signals (notshown). Such space switches are well known and include electricallycontrollable elements of variable refractive index to produce selectiveswitching between the inputs and outputs.

The four optical channels of each input fibre 1-3 are applied to aninput of each of the space switches X₁ -X₄ and each is separatelyselected by means of a respective tunable filter F1₁, F2₁, F3₁, F4₁.Similarly, the various wavelength channels from fibres 2 and 3 areapplied to respective inputs of the space switches through filters F1₂-F4₂, F1₃ -F4₃. The fourth input of each space switch X₁ -X₄ isconnected to tunable optical transmitter units 7 which produce opticalsignals of a single frequency in response to digital electrical signalstreams from the lower electrical layer of the network, to allowelectrical signals to be added into the optical network for transmissionin the optical layer. The electrical signals may be SDH, PDH, ATM, X25or any other suitable format, since the optical network can transmit thebit stream transparently in the optical domain. Similarly the outputs hof each of the space switches X₁ -X₄ are connected to receiver units 8that include photodetectors for producing electrical signals, so thatdigital data streams can be dropped out of the optical network into thelower, electrical layer. The outputs e-g of the space switches X₁ -X₄are respectively connected to the inputs to the fibres 4, 5 and 6 bymeans of combiners 9, 10, 11. Thus, by applying control signals to thespace switches X₁ -X₄ traffic in the WDM channels of input fibres 1-3can be switched selectively between output fibres 4-6 and signal trainscan be added to and dropped out of the optical network by means of thetransmitters and receivers 7, 8. This arrangement however suffers from anumber of disadvantages.

Firstly, the ultimate size of the OXC is limited by the size of thespace switches. Currently, these are made in LiNbO₃ or InP and arelimited for practical purposes to 8×8. Also, for polarisationindependent operation they typically require high switching voltages ofthe order of +/-100 V and have long rise times >100 ns. Furthermore, thedevices are not at present hermetically sealed, and the material thereoftends to exhibit drift characteristics, so as to alter the performancewith time.

Secondly, a node configuration as shown in FIG. 1 produces a loss ofapproximately 30 dB and the main contributor to the loss is the switcharray. In order to overcome this problem, optical fibre amplifiers suchas erbium doped fibre amplifiers need to be used to provideamplification.

Thirdly, the node architecture exhibits blocking characteristics. It isnot possible for the same wavelength channel from two of the inputfibres to leave on the same output fibre.

SUMMARY OF THE INVENTION

The invention provides an improved cross-connect, suitable for anoptical network, which may overcome at least some of these problems. Inaccordance with the invention there is provided an optical cross-connectcomprising: a plurality of input means each for receiving a respectivegroup of input signals and deriving in response to each signal of thegroup an individual optical signal of a respective given wavelength, aplurality of output means each including a plurality of receptorsselectively responsive to respective ones of the wavelengths of saidoptical signals to provide an output signal for the output means, and aplurality of coupling means each for coupling a respective one of saidoptical signals, each of a different wavelength, from each of the inputmeans, to a respective receptor of each of the output means.

In accordance with the invention, it is possible to route signals of thesame wavelength from different input means co a particular one of theoutput means. Furthermore, the losses involved may be substantiallyreduced.

In one form, the plurality of coupling means comprises a plurality ofstar couplers, and in another form comprises a plurality of optical buslines to which the input and output means may be selectively added andremoved as modules.

The input signals may be electrical or optical and the plurality ofinput means may be configured to process optical and electrical signals,to allow electrical signals to be added into the optical signal traffic.Similarly, at least one of the output means may produce electricaloutput signals to allow optical signals to be dropped to an electricalnetwork.

Furthermore, an improved non-blocking arrangement may be provided, inwhich a plurality of the input means are each operative to receive arespective group (k) of input signals and derive in response to eachsignal of the group an individual optical signal of a respective givenwavelength, the individual output signals being provided on selectedones of a plurality of output channels greater than k, a plurality ofthe output means each including a plurality of receptors greater innumber than k, are selectively responsive to respective ones of thewavelengths of said optical signals to provide an output signal for theoutput means, and a plurality of the coupling means greater in numberthan k, are each operative to couple a respective one of said opticalsignals, each of a different wavelength, from each of the input means,to a respective receptor of each of the output means.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood an embodimentthereof will now be described by way of example, reference being had tothe accompanying drawings in which:

FIG. 1 is a schematic block diagram of a prior OXC, describedhereinbefore;

FIG. 2 is a schematic block diagram of an optical cross-connect inaccordance with the invention;

FIG. 3 illustrates schematically input and output modules of thecross-connect shown in FIG. 2;

FIG. 4 illustrates signal blocking in the arrangement of FIGS. 2 and 3;

FIG. 5 is a schematic block diagram of the cross-connect with analternative form of input module which avoids signal blocking;

FIG. 6 is a schematic block diagram of an alternative form ofcross-connect in accordance with the invention, which utilises a backplane bus;

FIG. 7 illustrates a further optical cross-connect in accordance withthe invention; and

FIG. 8 illustrates schematically input and output modules of thecross-connect shown in FIG. 7.

DETAILED DESCRIPTION

Referring to FIG. 2, the OXC in accordance with the invention isconfigured for use at a node in a WDM hierarchical telecommunicationsystem that includes an optical layer overlying various electroniclayers.

The OXC includes a number of input means in the form of input modules10₁ -10_(n) to which optical fibres 11₁ -11_(n) are connected, theoptical fibres carrying k channel WDX signals. The OXC also receivesinputs from the electronic layer that are applied to input modules 12₁-12_(n) '. Each input module 12 receives k electrical lines 13 carryingelectrical input digital signals.

As will be explained in more detail hereinafter, signals correspondingto the k channels of each electrical and optical input, are connected bymeans of k star couplers 14₁₋₁₄ _(k) to a plurality of output means inthe form of optical output modules 15₁ -15_(n) and electronic outputmodules 16₁ -16_(n) '. The output modules 15₁ -15_(n) provide k channelWDM optical signals on output fibres 17₁ -17_(n). The output modules 16provide electrical outputs that can be dropped down to the electroniclayer of the network on lines 18₁₋₁₈ _(n').

The structure of a first example of input module 10 is shown in FIG. 3Aand consists of a demultiplexer 19 which demultiplexes k channel WDMsignals on input fibre 11 onto k parallel channels, each of whichincludes a wavelength converter 20₁ -20_(k). The wavelength converterseach produce a respective different wavelength in response to the inputsignal applied thereto so that the device shown in FIG. 3A provides oneof N different output wavelengths to be coupled to the star couplers 14as will be described hereinafter, where N corresponds to the totalnumber of input or output modules (n+n'). The wavelength converters maycomprise all-optical devices or may include an opto-electrical detectorthat drives a tunable laser. Examples of specific wavelength converterscurrently available are listed in "Photonic terabit/s networks and theirkey components" H. R. van As, IBM Research Div (Ruschlikon) 11^(th)Annual Conference on European Fibre Optic Communications and Networks(EFOC & N '93) Jun. 30-July 1993 pp 13-20, and in particular on page 18.

The structure of each input module 12 for electrical signals is shown inmore detail in FIG. 3B. Each input module 12 receives k parallelelectrical lines 13₁ -13_(k) that are connected to respective lasers 21₁-21_(k) that are tunable over the N different output wavelengths. Thusthe lasers, which are typically laser diodes, produce output modulatedsignals at k different wavelengths, for the k input electrical signalchannels. Specific examples of commercially available tunable laserssuitable for the system are listed on page 17 of the EFOC & N '93 paper,supra.

Referring to FIG. 2, the star couplers 14₁ -14_(k) each include Noptical inputs and N optical outputs. The N inputs of each star coupler14 are connected to receive individual different frequency outputs fromeach of the input modules. Thus, referring to FIG. 2, star coupler 14₁has inputs connected to receive a respective different wavelength signaltrain from each of the input modules 10₁ -10_(n) and 12₁ -12_(n) '. Inorder to simplify the drawing, only the connections from input module10₁ and 12_(n) ' are shown. Similarly, for the star coupler 14_(k),connections from input module 10₁ and 12_(n) ' are shown with the otherconnections being omitted to simplify the drawing. The connections tostar couplers 14₂ -14_(k-1) have also been omitted for simplicity. Foreach of the star couplers 14, the inputs are so arranged that eachreceives an individual wavelength from the groups of differentwavelengths produced by each of the input modules 10 and 12.

Each star coupler 14 operates in a manner well known per se so as to mixall the input signal trains and feed them to all of its outputs. Thus,considering for example star coupler 14₁, a mixture of all of its inputsis applied to N outputs, namely to each of the output modules 15₁-15_(n), 16₁ -16_(n) '.

The nature of each of the output modules will now be described withreference to FIG. 3. Referring to FIG. 3C, the structure of each opticaloutput module 15 is shown. The module consists of k inputs 22₁ -22_(k)which are individually connected to one output of each of the starcouplers 14₁ -14_(k). Each input 22 is connected to a receptor in theform of a tunable filter 23₁ -23_(k) which is individually tunable inorder to select individual wavelengths from the mixture of signalsapplied to each input 22. Commercially available tunable filters aredescribed in detail on page 17 of the EFOC & N '93 paper, supra, and maycomprise electro-mechanically tuned filters (e.g. Fabry-Perot etalon),acousto-optical or semiconductor filters.

It will be recalled that each star coupler, connected to a correspondingone of the inputs 22, supplies a mixture of signals of differentwavelengths selected from one of the outputs of each of the inputmodules 10, 12. Thus, each tunable filter 23₁ -23_(k) can be used toselect an individual signal from one of the input modules on the basisof a wavelength selection. The tunable filters may be tuned individuallyby the application of an external control voltage from a control system(not shown). The output of each filter 23₁ -23_(k) is applied to acorresponding wavelength converter 24₁ -24_(k). The wavelengthconverters produce signals that are combined by means of a passivecombiner 25 to produce a k channel WDM signal on output fibre 17.

Thus, by suitable selection of the pass frequencies of the filters 23,selection of individual signals can be achieved for each output fibre 17so that a selective cross-connection can be achieved between the inputfibres 11 and output fibres 17. Moreover, electrical signals applied toinput modules 12 can be added into the k channel WDM signals produced atoutput fibres 17, thus lifting the electrical signal from the electroniclayer of the network to the optical layer.

FIG. 3D shows a structure for each electrical module 16, which allowsoptical signal trains to drop down from the optical layer to theelectrical layer of the network. Each output module 16 consists of kinput lines 26₁ -26_(k) which are applied to tunable filters 27₁ -27_(k)that function in a similar manner to the tunable filters 23 so as toselect individual signal wavelengths. The outputs of the tunable filters27 are applied to corresponding receivers 28₁ -28_(k), typically in theform of photodiodes, which produce electrical signals corresponding tothe signals of selected wavelength. The resulting k digital signaltrains are applied to k output lines, which make up each output 18.Thus, signals of particular wavelengths are selected by the tunablefilters 27 and are dropped down to electrical signals for use in theelectronic layer of the network.

A problem with the configuration described so far is that signalblocking can occur at some of the outputs 17. This can be understoodfrom FIG. 4 which is an enlarged, partial illustration of theconfiguration of FIG. 2, FIG. 3A and FIG. 3B, showing input modules 10₁,10₂, star coupler 14₁ and output module 15₁, the other components havingbeen omitted to simplify the drawing.

Referring to input module 10₁, the input WDM signal is demultiplexed bydemultiplexer 19₁ to provide k signals of wavelength λ₁ -λ_(k) which areapplied to respective wavelength converters 20_(1l) -20_(1k). Thewavelength converters produce respective outputs λ_(i) -λ_(m).

A similar set of signals is produced by input module 10₂, comprisingwavelengths λ_(j) 'λ_(n) as shown in FIG. 4. Each of the other inputmodules (not shown) produce similar outputs. Considering now the starcoupler 14₁, each of its N inputs receives a signal from the firstwavelength converter 20₁₁ -20₂₁, etc. and the arrangement is set up sothat each of the input wavelengths is different. Thus N separatewavelengths are applied to the star coupler. Accordingly, referring toFIG. 4, λ_(i) ≠λ_(j). As previously explained, the resulting mix ofsignals produced by star coupler 14₁ is applied to a single input ofeach of the output modules 15₁ -15_(n), 16₁ -16_(n'). Thus, consideringthe output module 15₁ as shown in FIG. 4, it is not possible to selectwavelength λ_(i) and λ_(j) from the input modules 10₁, 10₂simultaneously at the output module 15₁ since both of the signals areapplied simultaneously to one of the filters 23. In FIG. 4 this is shownas input filter 23₁. The filter 23₁ can select only one of thewavelengths λ_(i), λ_(j) and therefore, signal blocking of one of thewavelengths will occur.

This problem is overcome by the configuration shown in FIG. 5, in whicha different form of the input module 10' is provided. Referring to inputmodule 10'₁ the input WDM signals applied on input fibre 11₁ are fed toa beam splitter 30₁ that provides k parallel channel outputs each ofwhich is fed to a tunable filter 31_(1l) -31_(1k) that in turn areconnected to individual wavelength converters 20_(1l) -20_(1k). Thus, bytuning the individual filters 31_(1l) -31_(1k), the different inputsignals λ₁ -λ_(k) can be applied to different ones of the k channelsselectively. The wavelength converters 20_(1l) -20_(1k) then produceappropriate signals for the inputs of the star couplers 14. In FIG. 5,the first two star couplers 14₁, 14₂ are shown. When it is desired toassemble a signal at output module 15₁ that contains wavelength λ_(i)from input module 10'₁ and λ_(j) from input module 10'₂, if, aspreviously described with reference to FIG. 4, these signals wereapplied to the inputs of a single one of the star couplers, blockingwould occur. However, as shown in FIG. 5, the tunable filter 31₂₂ isadjusted such that wavelength converter 20₂₂ (rather than converter20₂₁) produces the output wavelength λ_(j). Consequently, the signal ofwavelength λ_(j) is applied to star couplers 14₂ and hence to the secondinput of the output module 15₁. Thus, the signals of wavelength λ_(i)and λ_(j) can be selected by the tunable filters 23₁ and 23₂respectively in the output module 15₁. In this way, signal blocking isavoided.

Referring to FIG. 6, this shows an alternative configuration in whichthe star couplers of FIG. 2 are replaced by a back plane bus. Thus, theindividual input modules 10₁ -10_(n), 12₁ -12_(n) have their k outputsconnected individually to k parallel optical fibres 32 which form afibre back plane. The output modules 15₁ -15_(n) and 16₁ -16_(n) havetheir respective k inputs selectively connected to the k fibres of theback plane. This arrangement is particularly suited to use with inputmodules of the form shown in FIG. 5 since individual signal trains canbe programmably applied to individual ones of the back plane fibres 32and then selectively received by the output modules 15, 16 under thecontrol of tunable filters 23, 27. This arrangement has the advantagethat the individual modules can be arranged as units that can be pluggedonto the back plane for example by using optical D-couplers.

The described examples of the invention have the advantage that thenumber of elements that need to be controlled is much smaller ascompared with the configuration shown in FIG. 1. Considering an exampleof the OXC of FIG. 2, when compared with that of FIG. 1, when the numberof wavelengths k=4 and the number of input and output optical lines is3, the component count is set out in the following table:

                  TABLE    ______________________________________                       FIG. 1                             FIG. 2    ______________________________________    Number of tunable filters                          12     16    Number of tunable sources                          4       4    Number of crosspoints                         112     --    Number of wavelength converters                         --      24    Total                128     44    ______________________________________

Furthermore, since the wavelength converters in the input modules can bepermanently set such that the output wavelength of a converter is alwaysthe same, the number of elements requiring control can be reduced to 36.

The optical star couplers of FIG. 2 and the back plane bus of FIG. 6 areinexpensive components as compared with the space switches shown inFIG. 1. Packaged optical star couplers which are wavelength insensitivebetween 1300-1600 nm and as large as 144×144 have been reported and arethus suited to implement the invention. Since the optical star couplersare themselves inexpensive compared to the input and output modulesshown in FIG. 2, it is envisaged that not all the input and outputmodules need be attached at the time of commissioning the system so thatthe system can readily be upgraded in terms of its capacity thereafter.The described input and output modules both include wavelengthconverters, with the advantage that if a signal inversion occurs at thewavelength converter, two inversions occur, so that no signal inversionwill occur in the final output of the cross-connect.

An embodiment of the invention will now be described in A which analternative way of avoiding channel blocking is achieved. Referring toFIG. 7, the structure is generally similar to that shown in FIG. 2 andlike parts are given the same reference numbers. Each of the inputoptical fibres 11 carries k optical multiplex channels, but each inputmodule 10, instead of providing k parallel channels, provides Q channelswhere:

    Q>2k-1                                                     (1)

Similarly, the input modules 12 provide Q parallel channels and Q starcouplers 14 are provided. Each of the output modules 15, 16 is providedwith Q parallel channels.

The various input and output modules 10, 12, 15 and 16 are shown in moredetail in FIGS. 8a-d respectively. Referring to FIG. 8a, each inputmodule 10 is similar to that shown in FIG. 5 but with Q channels, andconsists of an input fibre 11 that carries k input multiplexed channels,connected to a passive splitter 19 that has Q outputs coupled to Qtunable filters 31₁ -31_(Q) which are respectively connected towavelength converters 20₁ -20_(Q), in order to provide Q paralleloutputs. From the foregoing, it will be seen that the channel redundancyof (Q-k) and the tunable nature of the various channels Q allows astrictly non-blocking configuration to be provided.

Referring now to FIG. 8b which shows the input module 12, thearrangement has Q input channels connected to respective tunable lasers21₁ -21_(Q). For a system which provides k input channels, the remaininginput channels (Q-k) can be used to provide test signals. Theconfiguration of the output modules 15, 16 shown in FIGS. 8c, d is thesame as that shown in FIGS. 3c, d except for the provision of Q parallelchannels rather than k. It will be seen that by appropriate selection ofthe wavelength of operation of tunable filters 31 and 23, 27, a fullynon-blocking architecture is provided by virtue of the (Q-k) channelredundancy. Whilst this non-blocking arrangement has been described foruse with the star couplers 14, it will be appreciated that it can alsobe used in connection with a back plane bus as shown in FIG. 6, bututilising Q back plane channels. Thus the various modules of FIG. 8,could be used with a Q channel back plane bus.

Since the wavelengths used in the cross-connect according to theinvention are selected independently of the wavelengths used in theoptical network itself, it is not necessary to control the precisenessof wavelength throughout the entire network but only on a link-by-linkbasis, since each optical cross-connect in accordance with the inventionprovides a decoupling between adjacent sections of the network.

Also, the separation of the N wavelengths used in the opticalcross-connect need not be the same as the separation of the wavelengthsλ₁ -λ_(k) used on the link. For example, the link wavelengths λ₁ -λ_(k)may each be separated by 2 nm whereas the wavelengths λ_(i), λ_(n) canbe separated by 1 nm, 2 nm, 3 nm or any other suitable spacing.

Thus, the set of wavelengths used in the optical cross-connect isindependent of the network and the number of trunk lines and the numberof channels k used in the network itself. Thus, N can be chosenindependently of k.

I claim:
 1. An optical cross-connect comprising:a plurality of inputmodules each configured to receive a respective group of input signalsand derive in response to each signal of the group an individual opticalsignal (λ₁) of a respective given wavelength, a plurality of outputmodules each including a plurality of receptors selectively responsiveto respective ones of the wavelengths of said optical signals to providean output signal for the output module, and a plurality of couplers eachconfigured to couple a respective one of said optical signals, each of adifferent wavelength, from each of the input modules, to a receptor ofeach of the output modules, wherein said input modules and said outputmodules are selectively connectible to said couplers.
 2. An opticalcross-connect according to claim 1 wherein said input signals comprisemultiplexed optical signals and at least some of said input modulesinclude elements responsive to the multiplexed optical signals toproduce said individual optical signals of respective given wavelengths.3. An optical cross-connect according to claim 2 wherein saidmultiplexed optical signals comprise WDM signals.
 4. An opticalcross-connect according to claim 3 wherein the input modules comprise aWDM demultiplexer for demultiplexing the input WDM signals, and aplurality of wavelength converters responsive respectively to thedemultiplexed signals for producing said individual optical signals. 5.An optical cross-connect according to claim 3 wherein said input modulesinclude selection elements for determining which one of said individualoptical signals from the respective input modules is to be coupled to aparticular one of the couplers.
 6. An optical cross-connect according toclaim 5 wherein at least some of said selection elements include a beamsplitter for splitting the WDM optical signals into a plurality ofpaths, and tunable filters associated with the paths for producing anindividual optical signal on each path.
 7. An optical cross-connectaccording to claim 6 including wavelength converters connected to theoutputs of the tunable filters respectively.
 8. An optical cross-connectaccording to claim 2 wherein at least some of the output modules includea respective combiner for combining the outputs of the receptors toprovide the output signal.
 9. An optical cross-connect according toclaim 8 including wavelength converters connected to the outputs of thereceptors.
 10. An optical cross-connect according to claim 1 wherein atleast one group of said input signals comprise electrical signals andthe input module that receives the electrical signals includes aplurality of tunable lasers operative to generate corresponding opticalsignals at said different wavelengths.
 11. An optical cross-connectaccording to claim 1 wherein at least one of the output modules includesphotodetectors connected to the receptors thereof for producing aplurality of output electrical signals.
 12. An optical cross-connectaccording to claim 1 wherein the receptors comprise tunable filters. 13.An optical cross-connect according to claim 1 wherein said plurality ofcouplers comprise a plurality of star couplers.
 14. An opticalcross-connect according to claim 1 wherein said plurality of couplerscomprise a plurality of bus lines, with the optical signals of differentwavelengths from each input module being coupled to the bus lines, andthe receptors of each output module being coupled to the bus lines. 15.An optical cross-connect according to claim 14 wherein said input andsaid output modules are selectively addable to and removable fromconnection with the bus lines.
 16. An optical cross-connect according toclaim 1 wherein the wavelengths of the individual optical signals areselectable independently of the wavelengths of the input signals.
 17. Anoptical cross-connect comprising:a plurality of input modules eachconfigured to receive a respective group of input signals and derive inresponse to each signal of the group an individual optical signal (λ₁)of a respective given wavelength, a plurality of output modules eachincluding a plurality of receptors selectively responsive to respectiveones of the wavelengths of said optical signals to provide an outputsignal for the output module, and a plurality of couplers eachconfigured to couple a respective one of said optical signals, each of adifferent wavelength, from each of the input modules, to a receptor ofeach of the output modules, wherein said input modules and said outputmodules are selectively connectable to said couplers; wherein aplurality of the input modules are each operative to receive arespective group (k) of input signals and derive in response to eachsignal of the group an individual optical signal of a respective givenwavelength, the individual output signals being provided on selectedones of a plurality of output channels greater than k, a plurality ofthe output modules each including a plurality of receptors greater innumber than k, are selectively responsive to respective ones of thewavelengths of said optical signals to provide an output signal for theoutput module and a plurality of the couplers greater in number than k,are each operative to couple a respective one of said optical signals,each of a different wavelength, from each of the input modules, to arespective receptor of each of the output modules.
 18. An opticalcross-connect according to claim 17 wherein the input modules eachinclude Q channels where Q>2k-1.
 19. An optical cross-connect accordingto claim 18 including Q of said couplers.
 20. An optical cross-connectaccording to claim 17 including Q of said receptors for each of saidoutput modules.
 21. An optical cross-connect comprising:a plurality ofparallel star couplers, each star coupler comprising a plurality ofinputs and a plurality of outputs, said inputs being configured toreceive a plurality of optical input signals, each at a differentwavelength, and to produce at each of said plurality of outputs of eachstar coupler a composite signal comprising all of the optical inputsignals input to that star coupler; a plurality of output modules, eachoutput module comprising a plurality of receptors; an output from eachof the plurality of parallel star couplers being coupled to each of saidoutput modules, said receptors being selectively responsive torespective ones of the wavelengths of said optical signals.
 22. Anoptical cross-connect according to claim 21, wherein each star couplerincludes at least one unused input, so as to produce an expandableoptical cross-connect.
 23. An optical cross-connect according to claim21, wherein each star coupler includes at least one unused output, so asto produce an expandable optical cross-connect.
 24. An opticalcross-connect comprising:a plurality of bus lines, each bus line beingconfigured to receive a plurality of optical input signals, each at adifferent wavelength, so as to produce at each bus line a compositesignal comprising all of the optical input signals input to that busline; a plurality of output modules, each output module comprising aplurality of receptors; each bus line being coupled to each of saidoutput modules and said receptors being selectively responsive torespective ones of the wavelengths of said optical signals.
 25. A methodof expanding an optical cross-connect, the optical cross-connectcomprising:a plurality of input modules each configured to receive arespective group of input signals and derive in response to each signalof the group an individual optical signal (λ_(i)) of a respective givenwavelength; a plurality of couplers comprising a plurality of inputs anda plurality of outputs, said input modules being connected to the inputsof said plurality of couplers; a plurality of output modules eachincluding a plurality of receptors selectively responsive to respectiveones of the wavelengths of said optical signals to provide an outputsignal for each output module, said output modules being connected tothe outputs of said plurality of couplers; and at least one additionalinput module comprising a plurality of outputs; wherein the plurality ofcouplers are each operable to couple a respective one of said opticalsignals, each of a different wavelength, from each of the input modules,to a receptor of each of the output modules, and each coupler includes anumber of inputs greater than the number of input modules such that anumber of said inputs are unconnected; the method comprising: connectingeach of said outputs of said at least one additional input module to anunconnected input on each of the couplers.
 26. A method of expanding anoptical cross-connect, the optical cross-connect comprising:a pluralityof input modules each configured to receive a respective group of inputsignals and derive in response to each signal of the group an individualoptical signal (λ_(i)) of a respective given wavelength; a plurality ofcouplers comprising a plurality of inputs and a plurality of outputs,said input modules being connected to the inputs of said plurality ofcouplers; a plurality of output modules each including a plurality ofreceptors selectively responsive to respective ones of the wavelengthsof said optical signals to provide an output signal for each outputmodule, said output modules being connected to the outputs of saidplurality of couplers; and at least one additional output modulecomprising a plurality of inputs; wherein the plurality of couplers areeach operable to couple a respective one of said optical signals, eachof a different wavelength, from each of the input modules, to a receptorof each of the output modules, and each coupler includes a number ofoutputs greater than the number of output modules such that a number ofsaid outputs are unconnected; the method comprising: connecting anunconnected output on each of the couplers to each of said inputs ofsaid at least one additional output module.
 27. In an opticalcross-connect comprising:a plurality of input modules each configured toreceive a respective group of input signals and derive in response toeach signal of the group an individual optical signal (λ_(i)) of arespective given wavelength, a plurality of output modules eachincluding a plurality of receptors selectively responsive to respectiveones of the wavelengths of said optical signals to provide an outputsignal for the output module, and a plurality of couplers eachconfigured to couple a respective one of said optical signals, each of adifferent wavelength, from each of the input modules, to a receptor ofeach of the output modules, wherein said input modules and said outputmodules are selectively connectible to said couplers; the improvementcomprising each of said couplers having at least one unconnected input,whereby an additional input module is connectible to said couplers so asto expand the cross-connect without increasing the number of couplers.28. In an optical cross-connect comprising:a plurality of input moduleseach configured to receive a respective group of input signals andderive in response to each signal of the group an individual opticalsignal (λ_(i) of a respective given wavelength, a plurality of outputmodules each including a plurality of receptors selectively responsiveto respective ones of the wavelengths of said optical signals to providean output signal for the output module, and a plurality of couplers eachconfigured to couple a respective one of said optical signals, each of adifferent wavelength, from each of the input modules, to a receptor ofeach of the output modules, wherein said input modules and said outputmodules are selectively connectible to said couplers; the improvementcomprising each of said couplers having at least one unconnected output,whereby an additional output module is connectible to said couplers soas to expand the cross-connect without increasing the number ofcouplers.